A key sheet is a component (subassembly) constituting a keyboard portion of a handy mobile equipment such as a portable phone, a personal digital assistant (PDA) or the like, and is configured such that a plurality of key tops made of a resin which are push buttons indicating a letter and/or a numerical key, a function key or the like respectively were adhered in an arrayed manner onto the top surface of a sheet referred to as a keypad. In many cases, polycarbonate or the like is usually used from the viewpoint of mold-ability, strength or the like as the above-mentioned resin material for the key top. Further, a soft material having rubber elasticity such as a silicone rubber, thermoplastic elastomer or the like is used as a keypad material. On the undersurface of the key sheet thus configured, a small projection referred to as a pressing element (a contact pressing projection) is provided for each of the above-mentioned key tops.
This key sheet is closely disposed on a printed circuit board provided with a plurality of normally open contacts. When any of the key tops arrayed on the top surface of the key sheet is pressed, a pressing element provided on the undersurface of the key sheet presses down a metal dome disposed at the underneath thereof, so that the normally open contact provided on the printed circuit board is closed. Therefore, a closed circuit corresponding to the pressed key top is formed.
Since the key sheet is thus configured such that a plurality of components are piled up in a layered manner, the thicknesses of each component is accumulated and it has substantial thickness. Therefore, it is requested that a whole handy mobile equipment should be further thinned so as to further enhance the portability thereof.
However, in a case where a resin is used for a key top as in the conventional key sheet, there is limitation for thinning a key top as one method for thinning an entire key sheet. For example, even in a material which is strong and excellent in impact resistance such as a polycarbonate resin, 0.5 mm to 0.6 mm is the thinnest limit in order to satisfy various conditions of strength required for a key top. Therefore, it can be considered to use a metal which is more excellent in strength than a resin and does not break even by applying excessive impact.
The following example is a conventional one using a metal for a key top, although it is different from the above-mentioned object. For example, in methods as disclosed in Patent References 1 and 2, even though a thin metal plate was used, only a flange portion of a key top is slightly formed out of a metal, and a resin was still used for other portions. Therefore it has simply a configuration of the conventional common layered structure formed by superposing a key top or the like made of a resin on a key pad. Therefore, it could not be expected to improve the strength of a key top, and in addition there is limitation for further thinning taking its structure into consideration.
[Patent Reference 1]
Japanese Laid-Open Patent [Kokai] Publication No. H-08-007691
[Patent Reference 2]
Japanese Laid-Open Patent [Kokai] Publication No. H-09-082174
When attempting to metalize an entire key top using a thin metal plate in order to improve the mechanical strength, another problem occurs. That is, it cannot be said that a metal has good adhesiveness to other materials such as a silicone rubber, a resin or the like, so that there is a case where sufficient adhesive strength could not be obtained. In addition, a metal cannot have translucency even though it try to be thinned. Therefore, in order to display letters, symbols or the like with a backlight, a hole penetrated from the front surface to the back surface for transmitting light in the shapes of letters, symbols or the like, so called letter holes must be carved. The edge of the carved letter holes would spoil the finger feeling or hand feeling of an operator who operates the key tops.